Basic DCE pass.

src/alloy/hir/block.h

@@ -16,6 +16,7 @@
 namespace alloy {
 namespace hir {
 
+class FunctionBuilder;
 class Instr;
 class Label;
 

src/alloy/hir/function_builder.cc

@@ -374,14 +374,17 @@ void FunctionBuilder::Comment(const char* format, ...) {
   i->src2.value = i->src3.value = NULL;
 }
 
-void FunctionBuilder::Nop() {
+const OpcodeInfo* FunctionBuilder::GetNopOpcode() const {
   STATIC_OPCODE(
       OPCODE_NOP,
       "nop",
       OPCODE_SIG_X,
       OPCODE_FLAG_IGNORE);
+  return &opcode;
+}
 
-  Instr* i = AppendInstr(opcode, 0);
+void FunctionBuilder::Nop() {
+  Instr* i = AppendInstr(*GetNopOpcode(), 0);
   i->src1.value = i->src2.value = i->src3.value = NULL;
 }
 

src/alloy/hir/function_builder.h

@@ -55,6 +55,7 @@ public:
 
   void Comment(const char* format, ...);
 
+  const OpcodeInfo* GetNopOpcode() const;
   void Nop();
 
   // trace info/etc

src/alloy/hir/instr.cc

@@ -47,3 +47,33 @@ void Instr::set_src3(Value* value) {
   src3.value = value;
   src3_use = value ? value->AddUse(block->arena, this) : NULL;
 }
+
+void Instr::Remove() {
+  if (dest) {
+    XEASSERT(!dest->use_head);
+    dest->def = NULL;
+  }
+  if (src1_use) {
+    src1.value->RemoveUse(src1_use);
+    src1_use = NULL;
+  }
+  if (src2_use) {
+    src2.value->RemoveUse(src2_use);
+    src2_use = NULL;
+  }
+  if (src3_use) {
+    src3.value->RemoveUse(src3_use);
+    src3_use = NULL;
+  }
+
+  if (prev) {
+    prev->next = next;
+  } else {
+    block->instr_head = next;
+  }
+  if (next) {
+    next->prev = prev;
+  } else {
+    block->instr_tail = prev;
+  }
+}

src/alloy/hir/instr.h

@@ -53,6 +53,8 @@ public:
   void set_src1(Value* value);
   void set_src2(Value* value);
   void set_src3(Value* value);
+
+  void Remove();
 };